Two million pound landing strings have been successfully manufactured and deployed. However, operators are setting larger diameter and heavier casing to ever-increasing depths requiring landing strings with increased setting capacity. Drilling rigs, top drives and associated equipment with capacity of 1,250 tons are in use. Landing strings with 2,500,000 pound capacity will be required by the drilling industry. This paper describes the design challenges of developing and manufacturing a 2.5 million pound landing string.
To meet the design objectives, 6-5/8 inch 150,000 psi yield strength pipe would require wall thickness of nearly 1-1/8 inches. To provide the required tensile capacity and decreased string weight, a new high-strength pipe with 165,000 psi specified minimum yield strength was developed. Slip-crushing resistance and elevator capacity requirements challenged existing manufacturing limits requiring unique designs and new high-strength materials in the slip-contact area.
The 6-5/8?? FH connection is a popular choice for landing strings. However, these higher load requirements have reached the limitations of the connection's ability to maintain shoulder engagement, provide a sealing mechanism, and maintain stress levels in the torque shoulder and counterbore below yield. Unique connection modifications and higher strength tool joints were required to meet performance objectives.
The challenges of developing a 2.5 million pound landing string are best appreciated by understanding the history of landing string development. First, a brief history of landing strings, from using the drill string available on the rig to using ultra-high capacity special purpose built strings, are reviewed. The development of the first 2 million pound slip-based landing string established the first published landing strings design methodology and design criteria.(15) This design criteria has been successfully used for several landing strings and is presented. As these criteria were applied to the design of the first 2.5 million pound landing string, several challenges had to be met. The design of each assembly component and the resolution of the challenges are described. In conclusion, successes, compromises, and lessons learned are presented.
Deep water and ultra-deep water well designs continue to drive the requirement for higher tension capacity landing strings. Water depth and total depth are increasing and step-outs are being extended, (Figure 1). This, combined with the often narrow margin between pore pressure, mud weight and fracture gradient, is causing well designers to set more intermediate casing strings and this in turn is pushing large diameter, heavy casing strings to deeper setting depths to maintain hole size and reach the intended hydrocarbon targets.
Initially, casing, liners, and offshore casing strings set in sub-sea wellheads were simply run on the drill pipe that was used to drill the well. The axial tension loads encountered created no significant concerns and were well within the capabilities of the standard drill pipe and pipe handling tools used to drill the well. As setting loads increased, systems that are more specialized were required for running these longer and heavier casing strings in increasing water depths. Initially, it was sufficient to simply increase wall thicknesses, increase slip segment lengths, increase material strengths, enlarge crosssectional areas, etc.; but eventually unique fit-for-purpose designs would be required.
